Slab mantle dehydrates beneath Kamchatka—Yet recycles water into the deep mantle

Konrad‐Schmolke, Matthias; Halama, Ralf; Manea, Vlad Constantin

doi:10.1002/2016gc006335

Cited by 37 publications

(28 citation statements)

References 91 publications

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“…Serpentine minerals often have high B concentrations, typically around 10-100 μg/g (Benton et al 2001;Vils et al 2008), which makes them a suitable source for high-B fluids. Thermodynamic-geochemical modelling also suggest that breakdown of serpentine during subduction can release B-rich, high-δ 11 B fluids (Konrad-Schmolke and Halama 2014; Konrad-Schmolke et al 2016). Moreover, the scenario of serpentinite-derived fluids fits with the Cignana peak P-T metamorphic conditions, which are similar to the expected antigorite breakdown (ca.…”

Section: Peak Metamorphic Fluid-rock Interactionmentioning

confidence: 66%

“…Consequently, slab dehydration is expected to lead to successively decreasing δ 11 B values in the dehydrating rocks of the subducting slab (Moran et al 1992;Marschall et al 2007;Konrad-Schmolke and Halama 2014). Whole-rock analyses of high-pressure metamorphic rocks have also shown that B correlates positively with H 2 O contents and traces progressive dehydration (Marschall et al 2009;Scambelluri et al 2004), which can be used to model across-arc variations in volcanic rocks that show systematic trends in B geochemistry (Konrad-Schmolke et al 2016).…”

Section: Introductionmentioning

confidence: 99%

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Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

Halama

Konrad‐Schmolke

Hoog³

2020

Contrib Mineral Petrol

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This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subductionrelated metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluidrock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 μg/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable (δ 11 B = − 10.3 to − 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in δ 11 B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive δ 11 B values (δ 11 B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 μg/g and δ 11 B values that cluster around 0% (δ 11 B = − 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.

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Section: Peak Metamorphic Fluid-rock Interactionmentioning

confidence: 66%

Section: Introductionmentioning

confidence: 99%

Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

Halama

Konrad‐Schmolke

Hoog³

2020

Contrib Mineral Petrol

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“…Some Li, Cl, and B can be incorporated into secondary minerals (e.g., zeolites, clays, chlorite, biotite, actinolite) during fluid-rock interaction, but in general, experimental and empirical data show that at temperatures <~150 °C, limited amounts of Li, Cl, and B are removed or added to the fluid either due to formation of secondary minerals or leaching of the host rock, respectively (e.g., Berger et al, 1988;James et al, 2003;Reyes and Trompetter, 2012;Seyfried et al, 1998Seyfried et al, , 1984. For Li, some experimental work has suggested that it can be released into the fluid phase with increasing temperatures from 25 to 250 °C despite the production of clay minerals (Millot et al, 2010), and at lower temperatures (<40 °C), B may be removed from the fluid by absorption onto clays (e.g., Keren and Mezuman, 1981;Palmer et al, 1987). Estimated subsurface temperatures for Hikurangi forearc cold and thermal springs are all <120 °C using K/Mg, K/Na, and silica geothermometry (Reyes et al, 2010), therefore modification of elemental ratios via fluid-rock interaction is thought to be limited, but we acknowledge that some modification is possible, particularly for lithium.…”

Section: Volatile Sourcementioning

confidence: 99%

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

et al. 2019

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In order to trace the cycling of fluid-mobile elements (FMEs) through subduction zone forearcs, we collected water samples from two warm and 16 cold springs along the subaerially exposed forearc of the Hikurangi subduction zone in New Zealand. Water samples were analyzed for their cation and anion concentrations, as well as their B, Li, Cl, and O stable isotope compositions. Fluids discharging through the prism have high concentrations of Cl (2400-16,000 mg/L), Br (6-70 mg/L), I (0.4-72 mg/L), Sr (0.1-200 mg/L), B (3-130 mg/L), Li (0.1-13 mg/L), and Na (33-6600 mg/L), consistent with data from previous studies. Most of these elements decrease overall in concentration from north to south, have a concentration peak in the central part of the margin, and have had limited concentration variability during the last three decades. Because Li, Cl, and B are all fluid-mobile elements, their incompatibility potentially limits modification by fluid-rock interaction, making them reliable tracers of fluid source. δ 37 Cl, δ 11 B, and δ 7 Li values range from −1.3‰ to +0.4‰ (n = 36), +11.8‰ to +41.9‰ (n = 25), and −3.1‰ to +29.0‰ (n = 29), respectively. Despite the change in concentrations along the margin, there is no corresponding trend in isotopic composition. Chlorine and boron isotope compositions are consistent with fluids dominated by seawater (δ 37 Cl = 0‰; δ 11 B = 40‰) and sedimentary pore fluids (δ 37 Cl ≈ −8‰ to 0‰; δ 11 B > ~17‰). Br/Cl (0.0025-0.005) and I/Cl (0.00005-0.007) weight ratios also support a dominant seawater and pore-fluid source. Lithium isotope data also suggest fluids sourced from seawater (δ 7 Li = +31‰) as well as dehydrating sediments and/or modified by interaction with local sediment. The fluid geochemical data cannot be explained by a change in the fluid source along the margin, but rather by a change in the upper-plate structural permeability. In the north, extension likely results in a highly permeable forearc, whereas transpression in the south traps fluids within the upper plate and dilutes saline fluids with groundwater. Such changes in upper-plate structural permeability may influence fluid pressure conditions within the forearc, which in turn may influence the observed change in slip behavior on the interface from north Hikurangi (aseismic creep) to south Hikurangi (deep locking). This work highlights the role the upper plate may play in the geochemical modification and transport of slab-derived fluids, and supports the long-standing but poorly documented assumption that seawater and pore fluids are expelled at shallow levels in the subduction zone (<15 km) and therefore play a limited role in the transport of FMEs to great depths in subduction zones. ■ INTRODUCTION Lithium, chlorine, and boron are all highly fluid-mobile elements (FMEs). Their incompatibility in rocks limits modification by fluid-rock interaction, thereby making them potentially excellent tracers of fluid source. Isotopic systems of all three elements have been used as a fluid tracer in subduction zones or to trace re...

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“…Dehydration reactions as serpentinite is carried to sub-arc depths lead to release of 11 B-enriched fluid, which is either released from the subducting slab beneath the arc 21 , 22 , or enriches the forearc mantle which is in turn dragged to subarc depths 23 , 24 . Recent thermal modelling has suggested that conditions along the slab interface, and above the subducting slab, are generally too hot for serpentine to persist to sub-arc depths 25 , 26 , and attention has therefore focused on serpentinisation of the upper slab mantle 6 , 7 , 27 , in particular during bend-faulting on outer arc rises 28 , 29 .…”

Section: Introductionmentioning

confidence: 99%

“…Increasingly sophisticated models of fluid and mobile component release in subduction zones have been developed 6 , 7 , 27 , 30 . However, the input boron concentration and δ 11 B used in these models for subducted slab serpentinites are based almost entirely on samples collected from ophiolitic analogues, or from the seafloor in the Atlantic 16 , 17 , 19 , 20 , 31 , which formed by slow-spreading.…”

Section: Introductionmentioning

confidence: 99%

No significant boron in the hydrated mantle of most subducting slabs

et al. 2018

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Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling.

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Slab mantle dehydrates beneath Kamchatka—Yet recycles water into the deep mantle

Cited by 37 publications

References 91 publications

Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid–rock interaction in white mica (Lago di Cignana, Western Alps)

The role of the upper plate in controlling fluid-mobile element (Cl, Li, B) cycling through subduction zones: Hikurangi forearc, New Zealand

No significant boron in the hydrated mantle of most subducting slabs

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